Conventionally, a process that has been most ordinarily adopted as a process for producing a superconducting acceleration cavity for accelerating charged particles such as electrons, positrons or protons at high frequencies is a process of selecting deep drawing, cutting or some other working appropriately to form plate-form niobium into main parts which constitute a cavity, and then bonding and integrating these with each other by electron beam welding, as illustrated in FIG. 1. This production process requires many working steps; thus, there exists a problem that costs for producing an acceleration cavity are inevitably increased up. Furthermore, there exists a basic problem concerned with accelerating performances since electron beam welding is frequently used. For example, when welding defects are present, in particular, such defects are present in the equator portion of a cavity, heat is often generated in welded sites. Thus, it is known that the heat hinders a high accelerating electric field. However, a process which should be alternative to this process and is for producing stable and excellent acceleration cavities has not been found out; thus, the process is most frequently used at present also. FIG. 2 illustrates an example of a single-cell type superconducting acceleration cavity produced by the above-mentioned process, which is frequently used at present also, and names of portions or sites.
As described in many patent documents, many production processes have been so far investigated and suggested in order to provide an economical superconducting acceleration cavity excellent in accelerating performance. For example, a process described in JP-A-60-261202 is a process wherein attention is paid to a problem that in previously existing techniques, an abnormally thick and expensive niobium material is used in light of a fundamental function of acceleration in an acceleration cavity. In other words, in order to make niobium thin, the process is a process of: using, as a core member, a pipe made of aluminum or an alloy thereof; forming a niobium thin film on the outer peripheral surface of the pipe and a copper thin film on the above-mentioned niobium thin film by sputtering; coating the above-mentioned copper thin film with copper thickly by electroplating; enlarging the pipe by bulge forming to swell the central portion thereof, thereby making the portion into a spherical form; and melting and removing the aluminum or the alloy thereof as the core member, thereby producing a superconducting acceleration cavity. This process has advantages that the niobium material can be saved and any bonding site based on electron beam welding can be eliminated. However, in this process, no considerations are made for pollution of the niobium surface generated at the time of removing the aluminum or the alloy thereof with an acid or alkali, the purity of the formed niobium film, and stress to which the niobium thin film is subjected by the pipe-enlarging working. In other words, the niobium film of 5 to 6 μm thickness, which is originally coated, cannot resist the pipe-enlargement, and further considerations are not entirely made for “creases” or “irregularities” of the niobium surface generated by the pipe-enlargement or the dissolution of niobium and the reduction in the niobium thickness by chemical polishing or electropolishing which is frequently carried out to remove the pollution of the niobium surface after an acceleration cavity is formed. Thus, the process is a process which cannot be practically used at all. Additionally, there are problems about costs such that an expensive large-sized vacuum film-forming apparatus for forming a niobium thin film and a copper thin film is indispensable.
In contrast with the fact that it is essential in the process of JP-A-60-261202 that the pipe-enlarging step is performed after the sputtering of a niobium thin film, JP-A-1-231300 describes that an aluminum alloy pipe or oxygen-free copper material is subjected to both of drawing work and pipe-enlarging work to form a cavity form, and subsequently an inner surface of the cavity is subjected to mirror finishing and the inner surface of the cavity is coated with niobium by RF magnetron sputtering, thereby forming a superconducting acceleration cavity. Thus, this process described in JP-A-1-231300 is a very practical process. However, the acceleration cavity itself originally has a spherical form, so that there is caused a problem about the evenness of the film thickness distribution of the niobium thin film obtained by sputtering. There is also caused a basic problem which affects performances, an example thereof being pinholes which are frequently encountered in the form of thin films. Furthermore, as well as the process of JP-A-60-261202, there has not yet been overcome a problem of the dissolution of niobium or the reduction in the thickness of niobium which follows chemical polishing or electropolishing of the inner surface of the cavity for the purpose of removing the surface pollution of the inside of the cavity. If the film thickness is made large under consideration of dissolution loss of the niobium by the chemical polishing or the electropolishing, there are caused not only a problem about the time for forming the film but also a problem about the flatness of the surface. Moreover, as well as the case of JP-A-60-261202, a large-sized and expensive vacuum film-forming apparatus is essential. Accordingly, the production process of JP-A-1-231300 cannot be a stable process for producing a superconducting acceleration cavity since the process has many practical evil effects and cannot give a high accelerating electric field from the viewpoint of performances.
A process described in JP-A-3-274805 is a process suggested in light of drawbacks of thin niobium film forming processes as described above, wherein a vacuum film-forming apparatus (a vacuum chamber) is used. The process does not adopt the method of forming a thin film of niobium, and is a production process of forming cavity parts from a niobium thin plate of 0.3 to 1.0 mm thickness by drawing work or pressing work, integrating the parts with each other by electron beam welding to make a cavity, and then depositing copper onto the outer peripheral surface of the niobium by electroplating or thermal spraying. As a specific process thereof, suggested is a production process of coating the surface of niobium firstly with gold having a thickness of 0.1 μm or more, heating the whole surface (at 300° C. for 1 hour) in a non-oxidizing atmosphere to form diffusion layers of gold and niobium so as to cause the niobium surface and the gold to adhere closely to each other, and coating the diffusion layers with copper having a thickness of 1 to 3 mm by electroplating or plasma spraying, thereby producing a superconducting acceleration cavity. This process is basically a process of making the used niobium material merely into a thinner form, and is basically equivalent to a conventional process for producing a cavity. Furthermore, gold is coated with the niobium surface by electroplating, the gold is allowed to be thermally diffused to adhere closely to the niobium, and subsequently a cavity is finally made by copper electroplating or copper powder spraying in a plasma manner. However, supplementary experiments by the present inventors have demonstrated that the formation of a diffusion layer of gold onto niobium is not observed at the above-mentioned temperature, and no effect of improving the adhesiveness is found out. Furthermore, it is technically impossible for copper electroplating or copper spraying to assure an even film thickness on the outer peripheral surface of a superconducting acceleration cavity which is largely undulating in the shape thereof. In conclusion, the process cannot become a low cost process which cancels the effect of a reduction in the amount used of niobium material from the viewpoint of the completion degree of the process or costs. Thus, it is doubtful that the process will be realized.
Meanwhile, in recent years, as disclosed in “Development of A Seamless Superconducting High-Frequency Acceleration Cavity Using A Niobium/Copper Clad Material”, pp. 12 to 15, July 2002 (Report of Grants-in-Aid for Scientific Research from Ministry of Education, Culture, Sports, Science and Technology of Japan), the following trial is being realized in the form of the development of a process for producing a seamless superconducting acceleration cavity. The trial is to simplify conventional processes of forming cavity parts from niobium material by deep drawing, cutting work or the like, and then bonding and integrating the parts with each other by electron beam welding; and to omit the expensive electron beam welding as much as possible in order to decrease costs and avoid problems descendent from welding defects, thereby attaining a high accelerating electric field. The so-called seamless acceleration cavity producing process, wherein such electron beam welded sites are decreased, is a process of using a niobium piping material (pipe member) as a starting material and forming a spherical shape peculiar to a superconducting cavity at a time by explosive forming, spinning forming, hydraulic bulge forming (hydraulic forming) or the like. Such a process is known as a known technique.
Out of the above, the forming process using the firstly-described explosive forming is a process of putting gunpowder inside a piping material and attaining the forming by pressure of an explosion. In the case of an superconducting cavity having a spherical shape, deforming pressure is applied to the inside of the niobium pipe at a moment; accordingly, only a result that the material is locally stretched is given. Thus, the thickness of the material is not even after the material is worked. Additionally, the process is involved in a serious problem that specific sites are cracked; thus, the process is not a useful process.
The secondly-described spinning forming is a process of using plate-form niobium and deforming the plate material while rotating the material along a surface of a mold member having a cavity shape, thereby working the plate material. This process makes it possible to produce a seamless cavity made of niobium and having no electron beam welded sites at least in the equator portion of the cavity; however, the inner face of the cavity is creased or cracked since the plate-form niobium is forcibly shape-worked along the surface of the mold member. Accordingly, it cannot be denied that after the formation of the cavity, surface polishing/removing work is considerably involved in order to remove the cracks or creases in the inner face. JP-A-2002-141196 is a suggestion example for producing a superconducting cavity by spinning forming.
The thirdly-described hydraulic bulge forming is a process of arranging a forming mold prepared in advance outside a seamless niobium piping material as a starting material, pushing and shortening the piping material from both ends thereof, and inserting niobium material into the mold while giving oil pressure to the inside of the piping material, whereby a spherical form is produced. This process is better than the above-mentioned two other processes although the process gives slight unevenness to the inner face of the cavity. The process is most satisfactory out of seamless cavity producing processes.
Since any of the above-mentioned seamless cavity producing processes is a process of forming a superconducting cavity directly from a niobium piping material, the processes are those improved toward an aim of decreasing electron beam bonding sites largely and attaining a high accelerating electric field. However, an acceleration cavity needs to satisfy structural requirements for a pressure vessel. As a result, the following problems are not overcome: a problem that niobium material, which is expensive, is used in a thick wall form; and a problem that a high electric resistance of niobium at normal temperature induces a local heat generation phenomenon (called a hot spot), which hinders a highly accelerating electric field at very low temperatures, to cause the quench of the superconductive state. Niobium material essentially has these problems. Japanese Patent No. 3545502 does not necessarily suggest a seamless cavity producing process, but discloses a cavity producing process to which a hydraulic bulge forming method is applied.
In order to avoid using expensive niobium material in an amount more than requires and to decrease the generation of hot spots, a new seamless cavity producing process is also being suggested. The process is that there is formed a piping material in which a metal inexpensive and good in thermal conductivity, such as copper, as a heat radiation stabilizing material is compounded into niobium on the outer peripheral region of a niobium material; and the resultant is used as a starting material.
In JP-A-2000-306697, the heat radiation stabilizing material is expressed as a good thermally-conductive material. The document discloses a seamless superconducting cavity producing process of inserting piping materials made of a good thermally-conductive material onto the outside and the inside of a seamless niobium piping material which is thinner than the good thermally-conductive material and does not have any electron beam bonded surface at all, forming a copper/niobium/copper composite piping material by a hot isostatic press bonding method (HIP method), and then subjecting this material to hydraulic bulge forming, thereby decreasing electron beam welded sites up to limitation. In this process, the role of the copper piping material, which is a cylinder inside the niobium piping material, is to prevent the niobium from deteriorating under high-temperature and high-pressure condition which accompanies the HIP bonding method. However, there remains a problem that after the end of the bulge forming, the copper piping material of the inner cylinder must be dissolved and removed with a chemical agent for dissolving copper, for example, nitric acid. Additionally, the HIP bonding method itself requires an expensive and special apparatus and further the method is basically batch working. Furthermore, the most serious problem when the HIP bonding method is applied to the production of the above-mentioned copper/niobium composite pipe is that when the inner cylinder, the niobium pipe and the outer cylinder are designed and formed in such a manner that the fitting crossing of the diameters can have a margin so as to attain the insertion of each of the cylinders and the pipe with ease, the bonding strength cannot be sufficiently kept. Accordingly, apart from the case of forming a composite piping material for a superconducting acceleration cavity having a short length in the axial direction, the HIP bonding is unsuitable for the process for producing a composite piping material for an ordinary superconducting cavity having a total length of 1 m or more.
Considering the problems of the HIP bonding method, JP-A-2002-367799 describes a process of heating a normally conductive metal material and niobium material to subject the materials to hot rolling, or hot-extruding a cylinder made of a normally conductive metal piping material and a cylinder made of niobium material together with a column while making the diameters thereof short, whereby the normally conductive metal material and the niobium material are integrated with each other to form a composite piping material for forming an acceleration cavity. However, this process is too complicated. Thus, apart from the case of carrying out mass production of clad element pipes, the process is unsuitable for the aim of lowering costs.